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Measurement of turbulent transport in premixed turbulent flames by ...

Measurement of turbulent transport in premixed turbulent flames by ...

Measurement of turbulent transport in premixed turbulent flames by

Measurement of Turbulent Transport in Premixed Turbulent Flames by Conditioned Particle Image Velocimetry (CPIV) S. Pfadler * , M. Czichos, F. Dinkelacker and A. Leipertz Lehrstuhl für Technische Thermodynamik Friedrich-Alexander Universität Erlangen-Nürnberg Erlangen, Germany Abstract One of the important issues for the numerical approaches of turbulent premixed combustion is the modelling of the unknown turbulent transport term, where the correlation between velocity and species (or temperature) fluctuation is described. Only few measurements are known here, since the necessary simultaneous vector-scalar measurements have required a complex combination of two dimensional laser measurement techniques in the past. The purpose of this paper is to introduce a much simpler approach for instantaneous, two-dimensional vector-scalar measurement technique for turbulent premixed flames and to show its applicability for the direct determination of the turbulent transport term. The technique is based on Conditioned Particle Image Velocimetry (CPIV) and the utilisation of digital image processing. It is applicable for turbulent premixed flames in the thin-flame regime. Introduction Nowadays used numerical methods and codes for the analysis of turbulent premixed combustion systems do not yet fulfill the desirable needs. Only the results of time and cost intensive Direct Numerical Simulations (DNS) where all time and length scales are resolved can be regarded as equal to physical reality. On the other hand the development of simplifying methods (e.g. Reynolds Averaged Navier Stokes methods, RANS), being applicable for industrial purposes, is closely connected with a decrease of accuracy of the predicted results. Moreover a lot of models are just applicable to special cases and special care has to be taken to use the correct case specific parameters. The complex interaction between turbulence and combustion reaction is still not yet adequately understood in order to be able to set up simplifying models. In this context the turbulent transport of the often used reaction progress variable c is controversially discussed. In most of the codes closure of this turbulent flux term is performed by a gradient transport assumption which is valid for non reactive flows. Experiments [1, 2] and results from DNS [3] have shown that the reality shows a transition from counter gradient to gradient behaviour of the turbulent flux for higher turbulence conditions. But the point of alteration is still hard to predict by the existing models. A reasonable improvement of simplifying approaches is thus closely connected with a systematic comparison of numerical and experimental results. Especially the aimed generality in terms of the basic conditions makes it necessary to validate the numerical predictions with experimental data. Thus from the experimental side the necessity arises to measure as much as accessible two dimensional data * Corresponding author: sp@ltt.uni-erlangen.de Associated Web site: http://www.ltt.uni-erlangen.de Proceedings of the European Combustion Meeting 2005 instantaneously to make the comparisons as significant as possible. A combination of several laser measurement techniques has been used for this purpose in the past. With higher complexity the practicability of the measurement set-up can be reduced to such an extend that the repeatability for substantial parameter studies cannot be granted sufficiently. Moreover, the combination of two different measurement techniques itself can arise problems when the basic measurement principles show counteracting influences on each others. In this paper the possibilities of a measurement technique is presented which is able to measure instantaneous two-dimensional velocity and density fields in turbulent premixed flames and thus gives access to turbulent flux data of the reaction progress variable c. As instantaneous density information are accessible for each measurement point and time it is possible to express all data not only as Reynolds- but also as Favre- and thus as density weighted averaged quantities which is of special interest for comparisons to numerical investigations. Theory Desirable quantities to be measured in order to validate numerical calculations are the reaction progress variable field, mean and RMS- velocity fields and composed quantities like the turbulent flux term of the reaction progress variable ρ u ′′ i c ′′ , where '' denotes the Favre fluctuation. Besides the turbulence- and the mean reaction-rate field ω& , the turbulent flux of the reaction progress variable requires closure by models which ideally should be able to be used for different flow, turbulence and flame conditions.

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